Paris -LRB- CNN -RRB- A traumatic event , such as the 9/11 attacks or the assault on Charlie Hebdo in Paris two weeks ago , can produce huge shifts in public opinion . A surge in patriotism , perhaps higher church attendances -- but one would not expect it to boost the work of an 18th century writer .

In France , and especially in Paris , the mood over the past two weeks has been subdued , but punctured by public displays of solidarity across faiths . In a famously fractious society , there is a sense of cohesion . The French seemed to have coalesced in reaching back into their history and the Enlightenment : They have started reading Voltaire .

One of France 's most renowned philosophers , Voltaire published his `` Treatise on Tolerance '' in 1763 . It was an appeal for religious tolerance , within and between faiths .

The French publisher Folio says sales of the `` Treatise '' have increased significantly since the Charlie Hebdo attacks . Between 2003 and the beginning of this year , Folio had sold 120,000 copies of the book -- roughly 10,000 a year . From January 12 to 14 , immediately after the rally in Paris in support of free speech and tolerance , some 7,000 copies were sold .

Folio said another 20,000 new copies would be made available to cope with demand . Online orders on Amazon and Kindle have also spiked .

When first published , the `` Treatise '' was a revolutionary creed , and one that landed Voltaire in trouble with the French government and especially powerful religious interests , such as the Jesuits .

His message has endured to become a cornerstone of the French republic , where the state and religion are formally and forcefully separated . But Voltaire went far beyond the rifts within Christianity in the `` Treatise . ''

`` I say that we should regard all men as our brothers , '' he wrote . `` What ? The Turk my brother ? The Chinaman my brother ? The Jew ? The Siam ? Yes , without doubt ; are we not all children of the same father and creatures of the same God ? ''

Voltaire wrote the `` Treatise '' because he was outraged by the execution of a Protestant man , Jean Calas , who was wrongly convicted of murdering his own son to prevent him from converting to Catholicism . His campaign to clear Jean Calas went all the way to King Louis XV , who pardoned Calas posthumously .

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Last week , the justice minister here , socialist Christiane Taubira , celebrated Voltaire 's legacy when speaking at the memorial for the Charlie Hebdo cartoonist Tignous .

`` We can draw anything , including a prophet , because in France , the France of Voltaire and irreverence , we have the right to make fun of religions , '' she said . `` A right . Yes , because a right is democracy and democracy is the realm of the law . ''

That sentiment was shared across the political spectrum . Writing in Le Figaro on Tuesday , former Prime Minister Dominique Villepin said that `` Faced with the drama that struck it , France has shown great dignity . ''

Villepin , from the conservative UMP , wrote that , `` The people have chosen by instinct loyalty to France . ''

It was fitting , and perhaps not accidental , that when world leaders gathered at the rally last week to commemorate those killed in Paris , they began their walk down the Boulevard Voltaire , toward the Place de la Nation . Scattered around them , among the many tributes , posters of the writer bordered with the motto : `` Je suis Charlie . ''

Some of the marchers waved copies of `` A Treatise on Tolerance '' as they walked and left them at impromptu memorials in the Place de la Republique .

The chateau at Versailles , once the residence of royalty , devoted the Hall of the Pope to a portrait of Voltaire in honor of the victims of the terror attack . Underneath , a sign quotes from the `` Treatise '' : `` What is tolerance ? It is the prerogative of humanity . ''

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The Societe Voltaire , charged with keeping the philosopher 's flame alive , says the attacks in Paris were also an attempt to assassinate him . In an article in L'Express , the Societe said Voltaire 's lifelong struggle was to `` crush the infamous '' -LRB- a saying with which he often ended his letters -RRB- and defend the victims of fanaticism .

In the words of the Societe 's Alain Sager : `` The border today is not between the religious and the atheist , between the Christian and non-Christian , Muslim and non-Muslim , between the Jewish and non-Jewish . It is between barbarism and civilization . ''

Among the millions of tweets in the wake of the Charlie Hebdo killings , not a few cited these words attributed to Voltaire : `` I do not agree with what you have to say , but I 'll defend to the death your right to say it . ''

Voltaire never actually said that . The phrase was invented by his English biographer many years later .

But he did write this , in `` A Treatise on Tolerance '' : `` The fewer dogmas , the fewer disputes ; the fewer disputes , the fewer miseries : If this is not true , then I 'm wrong . ''

Two-hundred-fifty years later , the words have new resonance .

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After Charlie Hebdo attacks , French find comfort in writing from 1763

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Voltaire 's `` Treatise on Tolerance '' is an appeal for religious tolerance , within and between faiths